The present invention relates to a method and apparatus using ultrasonic energy to detect the fluid in a closed vessel. The invention is particularly useful in identifying fluids flowing in a pipeline to detect interfaces between different fluids. While ultrasonic devices have been used in the past to determine fluid levels within closed vessels, they have not been used to identify the fluid in the vessel. Fluid level detection is accomplished by providing a series of transducers along the vessel and determining the fluid level by the signals from the various transducers.